This application is based upon and claims priority of Japanese Patent Application No. 2001-192909, filed in Jun. 26, 2001, the contents being incorporated herein by reference.
1. Field of the Invention
The present invention relates to a method of forming a polysilicon film employed in manufacturing a liquid crystal display device, an organic EL (Electroluminescence) display device, etc.
2. Description of the Prior Art
The liquid crystal display device has advantages such that the device is thin and lightweight, can be driven by low voltage and has small power consumption, and is widely employed in various electronic devices. In particular, since the active matrix liquid crystal display device in which the TFTs (Thin Film Transistors) are provided every pixel is excellent in the display quality that such device is equivalent to the CRT (Cathode-Ray Tube), such device is employed in the display of the TV, the personal computer, etc.
In recent years, in the liquid crystal display device, the polysilicon is employed as the operation layer of the TFT. Since the polysilicon has the higher carrier mobility than the amorphous silicon, not only the high-speed operation of the element can be achieved but also the element size can be reduced. Thus, the higher definition of the liquid crystal display device can be attained. Also, since the driver circuits such as the data driver, the gate driver, etc. can be constructed by the TFTs, the driver circuits and the display portion can be integrated on the same substrate. Accordingly, the reduction in the number of steps and the reduction in the number of parts can be achieved, so that a production cost of the liquid crystal display device can be reduced and also the reliability can be improved.
In the prior art, the polysilicon film is formed by the method described in the following. That is, first the amorphous silicon film is formed on the glass substrate by the plasma CVD method. Then, while irradiating the pulse laser beam output from the excimer laser onto the amorphous silicon film, the laser irradiation region is moved (scanned) in one direction. In doing so, the amorphous silicon film is fused by the laser irradiation and then crystallized on the portion located on the outside of the laser irradiation region to produce the polysilicon. In this manner, the polysilicon film is formed on the substrate.
However, in the polysilicon film formed by the above method, the grain size of the silicon crystal is relatively small. The carrier mobility of the polysilicon film depends largely on the grain size of the silicon crystal. If the grain size of the crystal is small, the carrier mobility is also small. Also, in the polysilicon film formed by the method in the prior art, the grain size of the crystal is largely varied, so that variation of the transistor characteristics is increased.
It is an object of the present invention to provide a polysilicon film forming method capable of manufacturing a TFT that permits a high-speed operation and has small variation in the characteristics.
If the laser irradiation region is scanned while irradiating the continuous wave laser (CW laser) onto the amorphous silicon film, the silicon crystal that is 10 to 100 times larger than the silicon crystal generated by the excimer laser can be formed. However, if it is tried to form the polysilicon film on the overall surface of the glass substrate having the size that is employed in manufacturing the liquid crystal display panel, failures such as peeling and others are generated because of unstable factors (the impurity, the tension, etc.) of the film.
If the amorphous silicon film is formed on the substrate, then the amorphous silicon film is formed into a plurality of island or ribbon-like shapes by etching the amorphous silicon film by the photolithography method, and then the laser irradiation region is scanned while irradiating the CW laser onto respective amorphous silicon films, the polysilicon film made of the silicon crystal having the large grain size can be formed while avoiding the temperature rise of the substrate.
However, according to the experiment made by the inventors of the present invention, it is found that, if it is tried to form the polysilicon film into the island or ribbon-like shapes by this method, peeling is ready to occur when the patterns of the silicon film are formed densely. Actually, if the driver circuit of the display device is formed by the TFT, the polysilicon film must be formed in the dense state.
Therefore, the inventors of the present invention examined the degree of peelings of the polysilicon film by changing variously the shape of the amorphous silicon film. As a result, it is found that the degree of peelings is largely changed depending on the shape of the amorphous silicon film. The present invention was made based on such experimental results.
In other words, in the present invention, if a width of the island or ribbon-like amorphous silicon film (a width of a rectangle in which the pattern of the amorphous silicon film is inscribed: which is similar hereinafter) before the CW laser irradiation is 30 xcexcm or less, a shape is formed to satisfy any one condition of (1) a top end shape of a pattern (a top end in irradiating the laser: which is similar hereinafter) is a convex shape, (2) a top end shape is a concave shape and consists of straight lines and has three corner portions at a top end side, and both angles of the corner portions on both sides of the top end shape are set to 45 degree or more, (3) a top end shape is a concave shape and consists of curved lines, and (4) a width of a top end portion is 25 xcexcm or less.
Also, if a width of the island or ribbon-like amorphous silicon film before the CW laser irradiation is in excess of 30 xcexcm, a shape is formed to satisfy any one condition of (1) a top end shape of a pattern is a convex shape and consists of straight lines, (2) a top end shape of the pattern is a convex shape and a radius of curvature of the top end portion is 100 xcexcm or less, (3) a top end shape is a concave shape and consists of straight lines and has three corner portions at a top end side, and both angles of the corner portions on both sides of the top end shape are set to 45 degree or more, (4) a top end shape is a concave shape and consists of curved lines, and (5) a width of a top end portion is 25 xcexcm or less.
Accordingly, the peeling of the polysilicon film caused by the CW laser irradiation can be suppressed, and the polysilicon film formed of the silicon crystal whose grain size is large and uniform can be formed. As a result, the thin film transistors that can execute the high-speed operation and have the uniform characteristics can be formed, the higher performance of the liquid crystal display device, the organic EL display device, etc. can be achieved, and also the manufacturing yield can be improved.
The reason that the peeling can be suppressed when the shape of the amorphous silicon film before the CW laser irradiation is set as above does not become apparent. But it may be considered that the reason is such that, if the top end shape of the pattern is a convex shape, the growth of the crystal starts from one spot, whereas if the top end shape of the pattern is a rectangle, the growth of the crystal starts from plural spots, and the large stress is generated in portions at which these grown crystals are brought into contact with each other, thus peeling is generated.